EP0466148B1 - Process for preparing a polymer of tetrafluoroethylene in aqueous suspension - Google Patents

Description

The invention relates to a method for producing granular that cannot be processed from the melt Tetrafluoroethylene polymer by suspension polymerization of tetrafluoroethylene and 0 to 0.6 mol% of a perfluorinated olefin or perfluoro (alkyl vinyl) ether as a modifying comonomer in the aqueous phase in Presence of radical initiators, before the polymerization is a mixture of tetrafluoroethylene and Inert gas is injected.

Suspension polymerization of tetrafluoroethylene (TFE) in the aqueous phase has long been known (US Pat. No. 2,393,967). It takes place in the presence of water-soluble initiators, such as for example of alkali or ammonium persulfates, -percarbonates, -perphosphates or -perborates, or of water-soluble redox initiators. Furthermore usually buffers and precipitants at Polymerization added. The air in the remaining gas space of the reactor is carefully removed and TFE is pressed in, the polymerization pressure between 4 and 30 bar can be. After the start of the polymerization usually while keeping the Polymerization pressure according to the arising Amount of polymer replenished with gaseous TFE.

The in this way by suspension polymerization TFE polymers obtained are usually as called granular PTFE. They only contain comonomers as a so-called modifier, that is, in such a way small extent that the TFE polymer Characteristic of the homopolymer PTFE, from which Melt not to be processable, maintains.

Granular PTFE differs from the second type of PTFE powders, the so-called fine powders, which Polymerization in the presence of fluorinated emulsifiers aqueous colloidal dispersions and their subsequent Coagulation can be obtained in terms of its Grain structure, specific surface, its powder and Processing properties and its Possible applications in principle.

The present invention is concerned with manufacturing and processing of granular PTFE. The specialist is known that the usual procedures directly from the Polymerization reactor obtained suspension polymers of the TFE in the form of fibrous, irregular and for the most processing purposes to coarse particles. Such raw polymer powders can because of their poor pourability, their low bulk density and their too large mean particle diameter, which in is usually over 1500 µm, only among the largest Difficulties to be processed. Mostly such crude polymers from grinding to medium Subjected to particle diameters of <200 µm, which also the mechanical and electrical properties of it Manufactured moldings can be improved.

Especially where the common ones Processing methods of granular PTFE according to the so-called press-sintering technique or after ram extrusion automatic dosing systems are used like this Common today is trouble-free Dosing with raw polymer powder not possible. The poor flowability leads to frequent constipation in the material flow of the dosage and the too low Bulk weight is a hindrance to adequate Capacity utilization of the plant, as per work cycle little material can be filled. The grain too coarse due to high porosity and insufficient smoothness of the Surface of objects made from it.

Processability has therefore been known for a long time such granular PTFE molding powder by so-called Agglomeration process in moving liquid media To improve the action of mechanical forces. As such liquid agglomerating media have been described as water, organic liquids that are able to wet PTFE amino and / or hydroxyl substituted alkanes or mixtures from water and largely insoluble in water organic Liquids. Prerequisite for one Agglomeration process is a previous grinding of the obtained crude polymer on a medium Particle diameter <200 µm. According to a common procedure, as described, for example, in US Pat. No. 2,936,301, such a grinding expediently takes place in one Gas jet mill, however, those achieved with it Flow properties for automatic dosing are not suffice. Before the grinding process is a washing and a Drying process required by the bad Transport properties is extremely hindered. Out for this reason it has become natural, drying and Another fine grinding of the granular TFE raw polymer To precede wet grinding by means of a cutting device, while washing the raw polymer and the Removal of the polymerization aids are effected. Nevertheless, this wet grinding step is a time consuming and measure susceptible to failure, which one would like to do without would. It is the object of the present invention to do this to enable an improved by the raw polymer Grain structure is awarded.

DE-A-23 25 562 describes molding compositions consisting of 99.95 mole% to about 99.50 mole% TFE units and about 0.05 mol% to about 0.50 mol% chlorotrifluoroethylene, the Manufactured so that the partial prints of the two Monomers are constant and on during the polymerization this gives copolymers with constant composition will. It was found that the polymers thus obtained have improved resistance to cold flow. over the influence of inert gas pressure on the morphology of the No information was given on polymer particles.

The object of the present invention is achieved by a method of the beginning mentioned type, which is characterized in that the Total pressure of the pressed mixture 5 to 50 bar and the Concentration of tetrafluoroethylene in this mixture 30 to 70 mol%, said concentration by appropriate replenishment of tetrafluoroethylene is kept in this range during the polymerization.

The suspension polymerization according to the invention Process is carried out with stirring and other usual conditions carried out in the aqueous phase. Well-known radical-forming and water-soluble initiators used are, especially the alkali and ammonium salts of Persulfates, percarbonates, also water-soluble peroxides, Acyl peroxides or alkali permanganates. Likewise Redox systems with one of the aforementioned peroxidic compounds, in particular Persulfate, as an oxidizing component and with a reducing component such as bisulfite, hydrazine, dithionite or a water-soluble diimine Nitrogen compound such as azodicarboxylic acid and their salts or azodicarbonamide. The polymerization can both in a slightly acidic and in a slightly alkaline environment respectively. When using permanganates advantageously used an acidic medium in all other initiators advantageously an alkaline Milieu. The polymerization can optionally be in the presence small amounts of buffer substances, such as in particular Ammonium salts, for example ammonium carbaminate,

Ammonium carbonate and ammonium oxalate. Known precipitants, such as, for example, borax or inorganic, water-soluble phosphates, may also be present. Furthermore, small amounts of heavy metal salts may be used to facilitate the start-up decay of redox systems in concentrations of 1 × 10 ^-5 to 40 × 10 ^-5 % by weight, based on the total liquor, in the form of the salts of, for example, divalent copper, or trivalent iron or trivalent chromium.

The polymerization of the TFE can be as pure Homopolymerization take place or small amounts can a modifying comonomer in the polymerization, namely 0 to 0.6 mol%, preferably 0 to 0.4, in particular - when present - 0.05 to 0.4 mol%, based on TFE in the gas space. The doing in that Polymer built-in amount of such a modifying Comonomers are so small that the Characteristic of PTFE - namely not from the melt to be deformable - is retained. Modifying Comonomers are perfluorinated olefins and Perfluoro (alkyl vinyl) ether, including those described in ω-position are substituted by H, preferably Hexafluoropropylene and perfluoro (propyl vinyl) ether.

The aqueous polymerization medium optionally contains advantageously also small amounts of telogen inactive perfluorinated emulsifiers, such as salts of Perfluorocarboxylic acids. The concentration of such However, emulsifiers should not exceed 30 ppm, it is preferably below 20 ppm. That amount is like that dimensioned low that the resulting polymer is not as colloidal dispersion remains in the aqueous medium.

The suspension polymerization takes place at one temperature from 5 to 90 ° C, preferably 10 to 40 ° C, these Temperature during the course of the polymerization in is kept essentially constant.

The polymerization by the process according to the invention is carried out in the presence of an inert gas. In doing so the total pressure resulting from pressing on TFE and Inert gas builds up before the start of polymerization, 5 to 50 bar, preferably 5 to 30 bar, with a Mixture is pressed in which the concentration of the TFE 30 to 70 mol%, preferably 30 to 65 mol%, of Total mix is. As usual, this becomes with the aqueous medium containing the polymerization ingredients contains, filled reaction vessel carefully beforehand from air or oxygen through repeated, alternating Evacuate and purge with nitrogen freed.

After the polymerization is started, TFE replenished, in a quantity that the Concentration of the TFE, based on the gas mixture, in mentioned range from 30 to 70 mol%, preferably 30 to 65 mol%. In a preferred one The embodiment of the method according to the invention is TFE essentially in the extent of consumption by itself Forming polymer replenished, the total pressure according to the decrease in the free gas space in the Reaction vessel rises, but within the specified Limits remain and the TFE concentration essentially remains constant. In a further preferred variant of the The inventive method is the make-up of TFE regulated so that the total pressure during the The polymerization time remains constant and the TFE concentration decreases during this time, however remains within the limits given above.

Are inert gases in the process according to the invention all gaseous elements or compounds that are against TFE, if necessary also against modifiers present Comonomers, and against the ingredients of TFE polymerization are inert and not in the Intervene polymerization. These are preferably noble gases, such as helium, argon, krypton or neon, carbon dioxide or also perfluorocarbon compounds such as perfluoromethane or -ethane, but especially nitrogen.

It should be noted that polymerization of TFE under Addition of inert gases is mentioned in US Pat. No. 2,394,243. The suspension polymerization of TFE in described there aqueous phase is said to be at extremely high pressures be performed. Pressures up to 35.2 bar (500 lbs./sq.in) are said to be still effective, however the pressures actually applied are much higher, namely in the range of 70 to 280 bar. Here are these Expressly to understand as the TFE partial pressure any added inert gas is only and referred to solely as a means of further increasing the pressure. The addition of inert gas is not critical. The pressures mentioned are the initial pressures, one Make-up of TFE during the polymerization is not intended.

About a certain concentration of the TFE to be observed or is an influence on the resulting polymer nothing said. It was therefore very surprising that in inventive method by the addition of inert gas the grain structure of the formed in a certain dosage TFE polymerization in an extremely advantageous manner being affected. A manufactured according to the usual method Suspension polymer of the TFE has a wide range predominant proportion of particles that are fibrous or have a thread-like shape or granules with a thread-like shape Are appendages, and this type of particle is for that poor transport properties of the powder in the Drying and grinding process responsible. This will in Drying process the flow through the dryer and thus the continuous task on the gas jet mill hindered, so that a high quality grinding very becomes problematic. Therefore the ballast is one Wet grinding stage in continuous processing processes inevitable.

The manufactured by the inventive method Product has significantly fewer such thread-like Particles or appendages and a much more even one Grain size and it is therefore in the workup much easier to handle. This leads to the fact that TFE polymer flows better out of the boiler when drained, is more washable and that on the step of wet grinding can be completely dispensed with, without the drying process and in the subsequent fine grinding in the gas jet mill Disturbances occur.

Furthermore, the grindability of the after TFE polymer prepared according to the method of the invention significantly improved. With the same grinding conditions using a gas jet mill at the after Products produced according to the invention lower coarse fraction observed, as from Table 2 evident.

When using an impact mill, for example one Zircoplex classifier mill from the company Alpine, gives itself to recognize better grindability in that at the same Energy consumption of the mill achievable per unit of time Throughput of quality product for that product according to the invention is significantly higher.

Furthermore brings that variant of the invention Procedure in which (with decreasing TFE concentration) the Total pressure is kept constant, another advantage: The molecular weight distribution of the formed TFE polymer is considerably wider, which means as is known, the properties of molded from it Products are influenced favorably.

The invention is illustrated by the following examples.

example 1

100 l of deionized water and 6 g of ammonium oxalate dissolved in it are poured into a 200 l enamel kettle. The boiler is freed of all oxygen by alternately evacuating and flushing with purified nitrogen and the temperature is set to 15 ° C. Then 7 bar nitrogen and 6 bar TFE are pressed in succession corresponding to a total pressure of 13 bar. The polymerization is started by metering in 150 mg KMnO ₄ , dissolved in 100 ml degassed water. TFE is fed continuously during the polymerization, in such a way that the concentration of TFE in the gas space in the boiler, which is reduced as a result of the polymer formed, does not essentially change. This is achieved by gradually increasing the setpoint of the TFE pressure regulator according to the gas volume decrease. The decrease in gas volume is calculated from the integrally measured amount of TFE that has flowed into the boiler and the density (ρ ≈ 2.3 g / cm ³ ) of the PTFE formed. As a result, the total pressure during the polymerization is constantly increased, namely after 1 kg of PTFE formed. The gas phase is composed of 54 mol% N ₂ and 46 mol% TFE. This gas phase composition remains essentially constant throughout the polymerization. When the run-in quantity of 40 kg is reached, the total pressure is approximately 1.2 times the initial pressure. After the above-mentioned run-in quantity of 40 kg has been reached, the polymerization is ended by closing the TFE valve and relieving pressure.

The kettle is flushed several times with N _2, thereby degassing the polymer. The suspension is roughly separated from the water using a sieve. The raw polymer is dried here, as in all the following examples, at 220 ° C. in a forced-air drying cabinet.

Example 2

As described in Example 1, the kettle is charged and, after evacuation and nitrogen flushing, pressurized with 7 bar N ₂ and 6 bar TFE and the polymerization is also started with 150 mg KMnO ₄ . In contrast to Example 1, the total pressure of 13 bar is kept constant by a regulated TFE make-up throughout the polymerization by setting the pressure control setpoint to this value. When the run-in amount of 40 kg has been reached, the polymerization is ended.

In this experiment, the gas phase at the beginning of the polymerization consists of 54 mol% N ₂ and 46 mol% TFE, at the end of the polymerization 65 mol% N ₂ and 35 mol% TFE.

Example 3

Operation as in example 1, but instead of nitrogen Highly purified carbon dioxide is used as an inert gas.

Example 4

Operation as in example 2, but instead of nitrogen highly purified carbon dioxide is used.

Example 5

Operation as in example 1, but instead of nitrogen argon is used as the inert gas.

Example 6

Operation as in example 1, but instead of nitrogen helium is used as the inert gas.

Example 7 (comparative example)

The kettle is charged as described in Example 1 and inertized. After evacuation, they become Removal of the flushing nitrogen pressed 6 bar TFE and the polymerization by adding 150 mg once Potassium permanganate started. The pressure is on the total polymerization time kept constant and after When the run-in quantity of 40 kg is reached.

Example 8

The experiment is carried out as in Example 2 by keeping the total pressure constant, with the difference that at the start of the polymerization the gas phase is composed of 25 mol% N ₂ and 75 mol% TFE at a total pressure of 8 bar. At the end of the reaction, the gas phase composition is 30 mol% N ₂ and 70 mol% TFE.

Example 9

The 200 l kettle is filled with 100 l of deionized water in which 40 g of (NH ₄ ) ₂ CO _{3 are} dissolved, rendered inert by alternately evacuating and flushing with N ₂ and the temperature is set to 70.degree. Then 10 g of PPVE are metered in and 5.5 bar of N ₂ and 10 bar of TFE are injected. The polymerization is started by metering in 3 g of ammonium persulfate, dissolved in 100 ml of degassed water, once. During the polymerization, PPVE is metered in gradually after every 1 kg of PTFE, about 1.5 g per kg of PTFE, a total of 60 g.

The total pressure is during the entire Polymerization time kept constant. After reaching one Intake amount of 40 kg is carried out by the polymerization Interruption of the TFE supply and pressure relief ended.

At the start of the polymerization the gas phase composition is 64.5 mol% TFE and 35.5 mol% N ₂ , at the end of the polymerization 57.4 mol% TFE and 42.6 mol% N ₂ .

Example 10

As example 9, with the difference that 8 bar N ₂ and 8 bar TFE are pressed into the kettle at the start of the polymerization. The total pressure of 16 bar is also kept constant over the duration of the polymerization and the polymerization is ended at a run-in amount of 40 kg. The gas phase composition is 50 mol% TFE and 50 mol% N ₂ at the start of polymerization, 40 mol% TFE and 60 mol% N ₂ at the end of the polymerization.

Example 11 (comparative example)

The polymerization is carried out as in Example 9 with the difference that no nitrogen is pressed, so the polymerization at one constant TFE pressure of 10 bar is carried out.

Example 12 (comparative example)

Working like example 11, but with the difference that the polymerization at a constant TFE pressure of 8 bar is carried out.

The dried raw polymer is in accordance with ASTM regulation D 1457-88 subjected to a dryer fabric analysis. Here a sieve arrangement from Haver & Boecker, type Haver EML 200-89 with a sieve set of 3000, 2000, 1800, 1400, 1000, 800, 600, 400 and 200 µm are used.

The results in Table 1 are listed according to ASTM standard D 1457-88 in a cumulative (integral) form. From these results, it is easiest to assess the influence of the grain size distribution on the d ₅₀ value, which is also given. The lowering of this value in the polymers obtained by the process according to the invention is mainly due to the significantly reduced proportion of fibrous particles in the raw polymer.

The grain size distribution and the grain shape can be combined in one certain extent due to the special stirring conditions and influenced by the wall roughness of the reaction vessel will. Therefore, all of the experiments listed in Table 1 are in the same kettle with the same stirring conditions Have been carried out.

The dried raw polymer is also without previous wet grinding over a dosing bunker over a Dosing screw in an air jet mill from Alpine, Type 315 AS, below for all examples and Comparative examples ground the same grinding conditions. The Grinding conditions are room temperature, inlet pressure 5 bar.

The dosing rate is in Examples 1 to 8 14.5 kg / h, in Examples 9 to 12 8.5 kg / h. The lower dosing speed in Examples 9 to 12 is required, as is known, the modified TFE suspension polymers are generally worse are grindable as the polymers that without the addition of Modifiers were made. There will be each 15 kg of raw polymer ground.

The ground product is made according to the ASTM standard mentioned subjected to a wet sieve analysis. This is a device from Haver & Boecker with a sieve set of 100, 75, 50 and 33 µm used.

Integrale Korngrößenverteilung des gemahlenen Produktes Beispiel Nr. >100 µm >75 µm >50 µm >33 µm <33 µm 1 0,2 0,4 4,0 18,0 82,0 2 0,5 1,0 5,0 21,0 79,0 3 0 0,2 0,6 1,2 98,8 4 0 0,2 0,8 1,6 98,4 5 0 0 0,5 0,8 99,2 6 0 0 0,1 0,4 99,6 7 1,0 4,5 11,0 36,0 64,0 8 1,0 1,8 9,1 30,3 69,7 9 0 0,5 4,6 14,6 85,4 10 0 0,2 1,3 7,6 92,4 11 0,3 2,2 11,4 28,1 71,9 12 0,2 1,2 7,5 23,1 76,9 The results of this wet screening are listed in Table 2, also in a cumulative form. This table documents improved grindability of the products of the process according to the invention by the increase in the proportion <33 μm and by the decrease in the proportion> 50 μm.

Integral grain size distribution of the ground product Example No. > 100 µm > 75 µm > 50 µm > 33 µm <33 µm 1 0.2 0.4 4.0 18.0 82.0 2nd 0.5 1.0 5.0 21.0 79.0 3rd 0 0.2 0.6 1.2 98.8 4th 0 0.2 0.8 1.6 98.4 5 0 0 0.5 0.8 99.2 6 0 0 0.1 0.4 99.6 7 1.0 4.5 11.0 36.0 64.0 8th 1.0 1.8 9.1 30.3 69.7 9 0 0.5 4.6 14.6 85.4 10th 0 0.2 1.3 7.6 92.4 11 0.3 2.2 11.4 28.1 71.9 12th 0.2 1.2 7.5 23.1 76.9

Claims (7)

1. A process for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer by suspension polymerization of tetrafluoroethylene and 0 to 0.6 mol% of a perfluorinated olefin or perfluoro(alkyl vinyl)ether as modifying comonomer in an aqueous phase in the presence of free-radical forming initiators, a mixture of tetrafluoroethylene and an inert gas being charged under pressure before starting the polymerization, wherein the total pressure of the charged mixture is 5 to 50 bar and the concentration of the tetrafluoroethylene in this mixture is 30 to 70 mol%, said concentration being kept within this range by an appropriate feed of tetrafluoroethylene during the polymerization.
2. The process as claimed in claim 1 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein the total pressure of the charged mixture is 5 to 30 bar.
3. The process as claimed in one or more of claims 1 and 2 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein tetrafluoroethylene is fed in during the polymerization at the rate at which it is consumed.
4. The process as claimed in one or more of claims 1 and 2 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein tetrafluoroethylene is fed in during the polymerization at such a rate that the total pressure remains constant.
5. The process as claimed in one or more of claims 1 to 4 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein the inert gas is nitrogen.
6. The process as claimed in one or more of claims 1 to 5 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein tetrafluoroethylene and 0.05 to 0.4 mol% of a modifying comonomer are polymerized.
7. The process as claimed in one or more of claims 1 to 6 for the preparation of a non-melt-fabricable granular tetrafluoroethylene polymer, wherein the modifying comonomer employed is hexafluoropropylene or perfluoropropyl perfluorovinyl ether.